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This Week in Science

Sunspots Before Your Eyes

Sunspots have a dark central region, the umbra, surrounded by a region of lighter radial filaments, the penumbra, along which there are outward horizontal mass flows that are still not fully understood despite their discovery 100 years ago. Now Rempel et al. (p. 171, published online 18 June; see the Perspective by Scharmer) present comprehensive numerical simulations of a pair of sunspots that show the development of an outer penumbra with systematic radial outflows along channels of nearly horizontal magnetic field in regions where the average field inclination is greater than 45 degrees. The outflows result from rising hot plasma that turns over and is guided outward by the strong and inclined magnetic field. The simulations reproduce observed properties throughout the entire penumbra and show that the penumbral structure and outflows in sunspots can be understood in terms of convective flow in a magnetic field with varying inclination.

Cooling Down

Some aerosols, such as sulfates, reflect solar radiation and have a cooling effect on climate, while others, such as black carbon, have a warming effect because they absorb solar radiation. The preponderance of reflective aerosols causes the net effect to be one of cooling, but the amount of cooling is uncertain, owing to a large difference in estimates of the effect of aerosols in global aerosol models compared with estimates based on observations. Myhre (p. 187, published online 18 June; see the Perspective by Quaas) uses a combination of observational data and modeling to reconcile the two approaches, drawing particular attention to relative increases in the aerosol fraction of black carbon. Taken together, the results suggest a best estimate of the cooling effect of aerosols that is 60% of previous reports.

Starved to Life?

Caloric restriction—reducing the calories ingested by around 30% of that of a normal, fit individual—leads to substantial increases in life span in experimental animals. In an extensive study of caloric restriction in primates, Colman et al. (p. 201) report that rhesus monkeys, which were subjected to caloric restriction as adults and followed for the last 20 years, show decreased mortality and delayed onset of age-related diseases when compared to normally fed control animals. If compliance with such a diet were not so difficult, many humans would be strongly tempted to enjoy the decreased incidence of brain degeneration, cardiovascular disease, diabetes, and cancer apparent in this population of monkeys.

Neurogenesis and Spatial Memory

The dentate gyrus of the hippocampus is one of two sites in the brain where new neurons are produced throughout life. Adult-born neurons integrate into the dentate gyrus circuitry and are thought to play a role in learning and memory. However, their contribution to hippocampal function remains unclear. Clelland et al. (p. 210) disrupted neurogenesis in mice and used two behavioral tasks to test for impairment in the formation of uncorrelated episodic memory representations. In one task, two arms were presented and the mice were rewarded for choosing the most recently visited arm in an earlier sequence; in the second task, animals were rewarded for choosing a certain location on a touch screen. Ablation of neurogenesis affected discrimination performance in both tasks but only when the arms or screen locations were close to one another. Neurogenesis is thus necessary for spatial pattern separation in the dentate gyrus.

Parkfield Tremors

Parkfield, California, sits on the San Andreas Fault near the end of a major historic rupture in 1857. Recent monitoring has resolved a change in the seismic tremor—small repeating earthquakes that have been occurring in certain parts of the fault. Nadeau and Guilhem (p. 191) now show that the tremors increased and periodic episodes began around the time of two moderate nearby earthquakes 4 years ago. Surprisingly, the tremor episodes have persisted, rather than decaying after the quakes, which may imply that there has been a step change in the state of stress on this part of the San Andreas Fault.

Not Reston at All

Reston ebolavirus is named, mistakenly perhaps, for Reston, Virginia, where it was discovered in the 1970s in imported macaques. After some alarm it was found not to be virulent in humans, uniquely among the ebola viruses, which are characteristically fatal causing a horrific spectrum of symptoms. Using a panviral detection assay, Reston ebolavirus has been rediscovered by Barrette et al. (p. 204) in domesticated pigs in the Philippines in association with other viruses that cause respiratory illness. The strains involved are closely related to the original macaque strain and, given how little variance there is among the viruses, it appears that it is freely circulating between these species possibly, like several other zoonotic viruses, having a reservoir in bats. Serological assays indicated that farm workers have become infected, although no obvious symptoms of human disease have been reported.

Toward Drug Development

The vast majority of pharmaceuticals on the market are based on natural product molecules originally derived from living organisms. When it comes to newly approved drugs, however, the situation looks rather different. Difficulties with high-throughput screening and laboratory synthesis of natural products have led drug companies to focus on libraries of synthetic compounds, despite their providing a much lower “hit rate.” Li and Vederas (p. 161) review methodologies that facilitate the screening, analysis, and synthesis of natural product molecules and their derivatives. Given these advances and the vast numbers of organisms and environments that remain to be explored for potential drug candidates, the current lull in newly approved drugs based on natural products will likely be temporary.

Size Matters

Cells of different types come in different sizes. Size is established by a trade-off of growth and division; as a result, the distribution of cell size in a population is held stable over time. A classic question in cell physiology is whether the growth rate of cells is constant over time or whether, as cells grow bigger during the cell cycle, they grow faster and faster. Using experimental and mathematical approaches, Tzur et al. (p. 167; see the Perspective by Edgar and Kim) show that the growth rate of mouse lymphoblastoid cells in culture is slow in the G1 phase and then increases to reach a constant exponential rate. Thus, there is an active size-control mechanism that limits size variation in animal cells.

Strolling Out on a Quantum Walk

In a random walk, a walker moves one step to the left or one step to the right depending on the outcome of a coin toss. The distribution between possible locations is well known and forms the basis for algorithms in information processing, describing diffusion processes in physics or biology, and has even been used as a model for stock market prices. Karski et al. (p. 174) use a single caesium atom trapped in a one-dimensional optical lattice to implement the quantum counterpart—a quantum walk. The coherence of a quantum system results in a departure from the classical picture, producing a distribution that is quite different that depends on the internal state of the atom. The results may have implications for search algorithms and quantum information processing protocols.

Topological Insulators

Topological insulators are a recently discovered state of matter, in which the bulk is an insulator while the surface is metallic with counterpropagating spin states. The surface states are protected by the topology, or structure, of the Fermi surface in the bulk gap and are described by a Dirac cone showing linear dispersion behavior meeting at the Dirac point. Chen et al. (p. 178, published online 11 June) provide a comprehensive photoemission study on Bi2Te3 showing that it too falls into the category of topological band insulators. Moreover, there is just a single surface state with a single Dirac point in the photoemission spectrum. The identification of a material with a single Dirac point removes the ambiguity arising from multiple surface states and provides an ideal test-bed to probe the physics of these exotic new materials.

Watching Graphite Breathe

Electron microscopy is best known for its capacity to resolve spatial features. In a variant of the technique termed electron energy loss spectroscopy (EELS), the kinetic energy dissipated as the electron probe beam penetrates a sample can be quantified to characterize the material's underlying electronic structure. Carbone et al. (p. 181) have achieved femtosecond time resolution in an EELS study of graphite, and thereby traced the correlated motion of electrons and nuclei when the sample is heated by laser irradiation. The laser pulse induces rapid compression and then expansion of the layers in graphite's sheet structure. Concurrently, EELS uncovers shifts in delocalized electronic excitations, or plasmons. Layer compression coincides with a population shift from surface to bulk (that is, interlayer) plasmons, with the reverse shift accompanying expansion.

Electrons Accepted Here

Methane is produced in large quantities in marine sediments during the breakdown of organic matter. Methane is a powerful greenhouse gas that plays a large role in the regulation of climate. Methane is also an energy source for the abundant anaerobic methanotrophs that consume most of it before it ever reaches the atmosphere. The anaerobic oxidation of methane in marine systems depends on the presence of sulfate, which acts as an electron acceptor and is often considered essential for the reaction to proceed. Beal et al. (p. 184) report that anaerobic methane oxidation in marine sediments can be facilitated by iron and manganese, as well as by sulfate. Thus, anaerobic methane oxidation using iron and manganese could have been an important methane sink, and energy source, for the early biosphere.

Shelling Turtles

In almost all vertebrates, the shoulder girdle (scapula) lies outside the ribs. The turtle is unique in that the carapace, the dorsal part of the shell, which is formed from the ribs, encapsulates the scapula. To understand the origin of the turtle-specific body plan, Nagashima et al. (p. 193; see the cover; see the Perspective by Rieppel) compared chicken, mouse, and the Chinese soft shelled-turtle, Pelodiscus sinensis. Modern embryos were studied via whole-mount immunostaining, three-dimensional reconstructions, and with markers for early skeletal precursors and compared with previously reported fossils. Initially, embryos of the three animals share a common developmental pattern, one that is likely to have been shared with their last common ancestor. This pattern, however, is modified in the turtle by a specific folding of its body wall during embryogenesis. This folding preserves some of the connectivity between skeletal and muscle elements but also produces new connections.

Generation of Swine Flu

As the newly emerged influenza virus starts its journey to infect the world's human population, the genetic secrets of the 2009 outbreak of swine influenza A(H1N1) are being revealed. In extensive phylogenetic analyses, Garten et al. (p. 197, published online 22 May) confirm that of the eight elements of the virus, the basic components encoded by the hemagglutinin, nucleoprotein, and nonstructural genes originated in birds and transferred to pigs in 1918. Subsequently, these formed a triple reassortant with the RNA polymerase PB1 that transferred from birds in 1968 to humans and then to pigs in 1998, coupled with RNA polymerases PA and PB2 that transferred from birds to pigs in 1998. The neuraminidase and matrix protein genes that complete the virus came from birds and entered pigs in 1979. The analysis offers insights into drug susceptibility and virulence, as well as raising the possibility of hitherto unknown factors determining host specificity. A significant question is, what is the potential for the H1 component of the current seasonal flu vaccine to act as a booster? Apart from the need for ongoing sequencing to monitor for the emergence of new reassortants, future pig populations need to be closely monitored for emerging influenza viruses.

Paradoxical Pain After Opioid Withdrawal

Opioids are widely used both by pain patients and substance abusers. Withdrawal from opioids can be complicated by opioid-induced hyperalgesia, a paradoxical amplification of pain by opioids. Drdla et al. (p. 207) detected a novel cellular opioid action that may help to explain this paradoxical pain: the induction of synaptic long-term potentiation (LTP) in pain pathways by opioids. In vitro and in vivo µ-opioid receptor agonist withdrawal induced robust LTP at synapses of nociceptive C fibers. This is in contrast to the presynaptic inhibition by opioids at these synapses. Because opioidergic LTP and acute synaptic depression are spatially and mechanistically distinct, it may be possible to treat opioid-induced hyperalgesia selectively without compromising opioid analgesia.

Let Me Present to You

The presentation of exogenous antigens by major histocompatibility (MHC) class I molecules is referred to as cross-presentation. Cross-presentation by dendritic cells plays a central role in the priming of cytolytic T lymphocyte responses to natural and vaccine antigens and also in the initiation of autoimmune diseases such as type 1 diabetes. A satisfactory cell biological model of cross-presentation is not available, which would be required to decipher the link, inherent in cross-presentation, between the secretory and the endocytic pathways. Saveanu et al. (p. 213, published online 4 June) identify an aminopeptidase, insulin-regulated aminopeptidase (IRAP), which interacts directly with MHC class I molecules. IRAP plays an exclusive and important role in MHC class I cross-presentation of receptor-targeted and phagocytosed antigens. In particular, a specific endosomal compartment, which carries IRAP as a unique marker, is implicated in cross-presentation of phagocytosed antigens.

Making the Choice

The importance of cytokine signals for the generation of specific blood lineages is well known. However, whether cytokines only influence survival and proliferation of cells that have already committed to one lineage, or if they also influence the lineage choice is unclear. For decades, researchers disputed whether lineage choice of hematopoietic progenitor cells be influenced by cell extrinsic cytokines. Using bioimaging approaches that allow long-term observation of all cells in progenitor cell cultures, Rieger et al. (p. 217) now demonstrate that certain cytokines can indeed instruct blood lineage choice.